A micro-strip shorted patch antenna with a compact electromagnetic band-gap structure (CEBG) is presented and a performance comparison is made with the conventional short-patched device of identical dimensions. Measured data shows a gain improvement of 3dB and a significant reduction cross polarization for the CEBG antenna. Meanwhile it proves that the design of the same resonant frequency can be attained under such a condition that the CEBG structure cell size is only about 40%～50% of conventional structure. It indicates that the suggested condition is significant in realizing the array of EBG of micro-strip phased antenna.